1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process. More specifically the present invention relates to an image forming apparatus utilizing an electrophotographic process wherein a photosensitive member in the form of a flat plate, a drum or an endless belt is subjected to charge deposition by a corona discharge and is then exposed to a light image of an original to selectively eliminate said charge thereby forming a latent image which is subsequently rendered visible by depositing toner particles having a charge of a polarity opposite to that of said latent image, said apparatus being capable of preventing image deterioration resulting from the time-dependent or environment-dependent changes in the charging characteristics of the photosensitive member.
2. Brief Description of the Prior Art
As the charging characteristics of the photosensitive member are subject to significant changes of a time-dependent nature and also resulting from variations in the environmental conditions such as circumferential temperature and humidity, the user of the image forming apparatus, for example a copier, has been required to control the image density by manipulating a copy density dial for controlling a diaphragm provided in the optical system.
As the number of copies increases, the charging characteristics of the photosensitive member gradually vary from the initial values and eventually exceeds the density adjustable range of the diaphragm control dial, thus necessitating the replacement of the photosensitive member with a new one. For this reason the photosensitive member has been of a limited service life and has required frequent replacements.
There have therefore been proposed various methods for stabilizing the image density by measuring the latent image potential on the photosensitive member and controlling the developing bias according to the measured potential. Among these methods, particularly simple is a method for stabilizing the image density by measuring and holding the latent image potential of a highlight area obtained by exposure of a white area of an original or of an original support table, and by controlling the developing bias by means of a developing bias amplifier in such a manner as to maintain a constant difference or a constant linear relationship between the potential thus held and the developing bias. However, in a copier of which the optical system is provided with a diaphragm for controlling the image density according to the background color of the original, a change in the aforementioned highlight potential caused by changing the amount of exposure by said diaphragm is unable to achieve its intended object as the developing bias is regulated by said developing bias amplifier so as to compensate for such change.